The original thin-client protocol for multiuser NT is Independent Computing Architecture (ICA), which Citrix developed for use with its NT-based WinFrame product. Over time, several companies (i.e., Insignia Solutions, Network Computing DevicesNCD, and Exodus Communications) implemented support for the X11 protocol in conjunction with multiuser NT servers. Thus, you had the choice of using ICA-based terminals with or without client-side applications, or X terminals with or without X terminal emulators to access Windows-based applications.
When Microsoft developed Windows-based Terminal Server, it did not implement either the ICA or X11 protocol. Instead, Microsoft chose the Tshare protocol it had been using with NetMeeting. By choosing this protocol, Microsoft immediately pushed all of the existing ICA and X11 clients out of the Terminal Server picture. However, Microsoft gave Citrix license to address these clients. Thus, Citrix developed the pICAsso add-on to Terminal Server that supports ICA. With pICAsso in place, ICA clients can access Terminal Server. Citrix is also closing in on enabling X11 access: Citrix recently signed an agreement with Insignia to acquire Insignia's X11 multiuser NT technology.
Currently, Terminal Server supports only Tshare out of the box. If you want support for ICA, you must purchase the pICAsso add-on from Citrix. And if you want X11 access, you have to wait until Citrix brings another add-on to the market or extends pICAsso to embrace X11.